1. Field of the Invention
This invention relates to a method of strongly affixing a synthetic resin film and a molding resin together directly.
2. Description of the Related Art
According to the prior art, the following two methods are available for affixing a molding resin to a synthetic resin film:
(1) The synthetic resin film is provided with small holes and the molding resin is molded so as to close the small holes from above and below, whereby the synthetic resin film is sandwiched by the molding resin. PA1 (2) The surface of the synthetic resin film to which the molding resin is to be affixed is coated beforehand with a bonding agent and the molding resin is molded so as to contact the surface that has been coated with the bonding agent, whereby the molding resin is bonded to the synthetic resin film.
According to the method (1), the structure obtained is one in which the synthetic resin film is embraced by the molding resin, not one in which the two are bonded together. As a consequence, the adhesion between the film and the resin is poor and in some cases there is the danger that a gap will be formed between the two or that the two will become displaced relative to each other. Further, since the synthetic resin film is provided with holes, a completely sealed structure is not obtained since the holes are not perfectly filled with the molding resin. This means that the synthetic resin film cannot be endowed with a perfect waterproofing function.
According to method (2), excellent adhesion is obtained since the synthetic resin film and molding resin are bonded together. However, the synthetic resin film must be coated with the bonding agent before the molding resin is molded. This increases the number of process steps correspondingly.
Further, in a case where the synthetic resin film and molding resin consist of transparent materials and light is made to pass through them, problems such as heat discoloration and light discoloration arise depending upon the material used for the layer of bonding agent. Specifically, if the layer of bonding agent changes color, there is the danger that the transmittance of light will decline or that the color of the transmitted light will change.
In a situation where the molding resin has been attached by the layer of bonding agent during the forming of the synthetic resin film having this layer, heating of the molded product causes the layer of bonding agent to soften and to lose some of its adhesiveness. Thus there is the danger that the molding resin will peel off the synthetic resin film owing to residual stress possessed by the synthetic resin film after forming.